1. Field of the Invention
This invention relates to a dust adhesion prevention system for an image scanning system, and more particularly to a system for preventing dust from adhering to a cover glass of a casing in which a deflector for deflecting a scanning light beam is contained.
2. Description of the Related Art
In an image scanning system using a raster scan of a laser beam, the deflector for deflecting a scanning light beam is generally contained in a casing having a cover glass which transmits the scanning light beam in order to prevent dust from entering the optical system including the deflector. This prevents dust or the like from adhering to the reflecting surface of the deflector such as a rotary polygonal mirror or reflecting surfaces in the optical system downstream of the deflector. When dust adheres to the reflecting surfaces in the optical system downstream of the deflector, streaks appear in a recorded image described later.
Further the cover glass serves to prevent the reflecting surfaces of the polygonal mirror from staining and to prevent air turbulence caused by the polygonal mirror rotating at high speed from affecting the overall optical system. Stain on the reflecting surface of the polygonal mirror lowers the reflectivity of the polygonal mirror and causes shading or the like. In the case of a color printer, stain on the reflecting surface of the polygonal mirror can cause nonuniformity in color. Accordingly it is preferable to provide such a scanning system with a cover glass.
However when such a cover glass accumulates dust, the scanning light beam is affected by the dust and the scanning performance deteriorates. That is, when the scanning light beam deflected by the deflector for raster scan passes through the cover glass, the intensity of the scanning light beam is reduced at the portion of the cover glass with the dust thereon and the density of the recorded image is reduced at the same place on the main scanning line, which results in streaking extending in the sub-scanning direction. A similar phenomenon occurs in an image reading system, and this is fatal for an image scanning system.
Accordingly when such streaking appears in an image, the system must be stopped for cleaning of the cover glass. However there are objects which cannot be scanned again, e.g., a stimulable phosphor sheet for use in radiography, and accordingly deterioration in performance of such an image scanning should be avoided to the utmost.
Especially in the case of a system where recording is effected continuously at high speed, there is produced a great loss before the operator recognizes the defect due to dust on the cover glass and stops the system.